Epilepsy is the most common neurological disorder in pregnant women. Epileptic seizures can be dangerous both for the mother and the foetus. Most pregnant women with epilepsy must use antiepileptic drugs. The aim of the treatment is to gain optimal seizure control while minimising the risk to the foetus (a potential teratogenic effect of antiepileptic drugs). The physiological changes in the pregnant woman’s body may affect the effectiveness of these agents. As pregnancy progresses, their serum concentrations decrease due to the increase in extracellular fluid volume, impaired intestinal absorption, increased renal clearance and enhanced hepatic metabolism. This is true for most antiepileptics, both old and new. Particularly significant fluctuations of serum concentrations are observed for new antiepileptics. The level of lamotrigine and levetiracetam decreases in the third trimester of pregnancy by even 50%, and the level of oxcarbazepine falls by 30–40%. The dosage of antiepileptic drugs should depend primarily on the clinical picture, and it is helpful to determine their serum levels during pregnancy. It is recommended to test the concentration of antiepileptic drugs before pregnancy, at the beginning of each trimester, during the last month of pregnancy and during the eighth week after delivery. The paper presents two cases of pregnant women who experienced a decrease in the level of new-generation antiepileptic drugs during pregnancy. The knowledge about the levels of these drugs was a valuable guide for optimising treatment.